This invention relates to a system for driving a thermal head employed in a thermal recording apparatus.
In a heat-sensitive recording apparatus or thermal transfer type recording apparatus, the thermal head is selectively driven to record video data. The driving of a thermal head is, in general, limited by the power supply capacity. Let us consider a thermal head which records a line with 1728 dots. Assuming that, in recording data, with this thermal head, a current of 40 mA is required per dot, when a line is recorded in two printing cycles (i.e., using a two-cycle printing system), it can be determined from simple multiplication that the power supply capacity must be 34.6 A. The provision of a power source having such a large capacity is not economical, and may prevent miniaturization of the apparatus.
In order to overcome these difficulties, a system in which the thermal head is driven in a divisional manner, with the number of cycles being determined from the ratio of the current required to allow all the heat generating elements of the thermal head to generate heat simultaneously to the power source capacity, has been proposed in the art. However, the system is disadvantageous in that the recording speed is made low because the thermal head is driven uniformly in a divisional manner.
In order to eliminate the above-described drawback, another thermal head driving system is known in the art in which the number of printing cycles is changed according to the number of dots to be printed. In this system, when the number of dots to be printed corresponds to a current value which meets or is less than the power source capacity of the recording apparatus, a two-cycle printing operation, for example, (the general recording operation) is carried out. When the number of dots to be printed would exceed the power source capacity, the printing operation is carried out using an increased number of cycles (four cycles, eight cycles, sixteen cycles and so forth) depending on the number of dots to be printed. Accordingly, in this system, it is impossible to only slightly change the number of printing cycles. Therefore, at worst, when the number of dots to be printed is increased merely by one, the number of cycles is doubled and accordingly the recording speed is reduced to half. That is, this system is low in power source usage efficiency and is insufficient in recording speed.